Oxidative Stress refers to increased free-radical oxidation and a loss of antioxidant defenses. Postmenopausal women have an increased risk for coronary heart disease and may be prone to oxidative stress as a result of declining estrogen levels. The relationship between estrogen and oxidative stress will be investigated by a 2-year, longitudinal, follow-up study comparing 40 postmenopausal women taking estrogen replacement for one-year with 40 age-matched, postmenopausal women not on estrogen replacement. Measured variables will include: lipoprotein oxidation products (conjugated dienes) antioxidants, pro-oxidants, diet, and physical activity. Descriptive statistics, multivariant analyses of covariance, and correlational statistics will be used in the data analysis.